Method for producing a solid ceramic fuel cell

ABSTRACT

According to a method for producing a solid ceramic fuel cell, a solid electrolyte layer is gas-tightly deposited on an electrode inside a coating chamber, using a plasma spraying technique. The pressure inside the coating chamber is set at less than approximately 15 mbar for this purpose. A coating material is powder form, preferably with a particle diameter of significantly less than 10 μm, is finely dispersed in the plasma jet in such a way that the individual particles are isolated from each other when they meet the electrode. This enables a very homogenous and impervious solid electrolyte layer to be deposited.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/EP01/09760 which has an Internationalfiling date of Aug. 23, 2001, which designated the United States ofAmerica and which claims priority on European Patent Application numberEP 00118769.9 filed Aug. 30, 2000, the entire contents of which arehereby incorporated herein by reference.

FIELD OF THE INVENTION

The invention generally relates to a process for producing a solid oxidefuel cell. Preferably, it relates to a process for producing one havinga solid electrolyte layer arranged between two porous electrodes.

BACKGROUND OF THE INVENTION

A fuel cell generally comprises two electrodes, namely an anode and acathode, between which an electrolyte layer is arranged. Oxygen ispassed over the cathode surface and a fuel is passed over the anodesurface. Ion exchange between the fuel and the oxygen takes place viathe electrolyte layer, so that a voltage is formed between anode andcathode.

One of the crucial factors with regard to the efficiency of a fuel cellis the electrolyte layer. On the one hand, it must have a good ionconductivity and on the other hand, it needs to be substantiallygas-impermeable, in order to prevent gas exchange between fuel andoxygen. Therefore, high demands are imposed on the electrolyte layer.

In what are known as solid oxide fuel cells (SOFCs), the anode and thecathode are formed from a porous ceramic material, between which a solidelectrolyte layer is arranged. Solid oxide fuel cells with a planargeometry, in which the anode and cathode run substantiallyplane-parallel to one another, are known. In addition, a cylindrical ortubular configuration is also known. A tubular fuel cell of this typecomprises a porous ceramic inner cylinder as the cathode, to which theelectrolyte layer is applied, followed by the anode as a casing. Toproduce electrical connection to the cathode, there is what is known asan interconnector, which is directly connected to the cathode, theelectrolyte layer and the anode being interrupted in the region of theinterconnector.

A large number of processes for applying the solid electrolyte layer areknown. For example, DE 196 09 418 C2 discloses the application of asuspension which contains solid fractions of the solid electrolytematerial to a planar electrode. Excess solvent is removed by reducingthe pressure on the opposite side of the porous electrode from thesuspension. The suspension has coarse and fine solid fractions, thecoarse solid fractions initially blocking the pores in the electrode andensuring good bonding between the electrolyte layer and the electrode.The fine fractions are then deposited on the coarse fractions. The solidlayer is dried and then sintered in order to form the solid electrolytelayer. This coating process requires subsequent sintering of theelectrolyte layer at a high temperature.

Moreover, DE 196 09 418 C2 discloses that it is known to produce theelectrolyte layer by electrophoresis or by tape casting.

An EVD (EVD: electrochemical vapor deposition) process for applying theelectrolyte layer is known from the article “Status of Solid Oxide FuelCell Technology” by S. C. Singhal, taken from: High TemperatureElectrochemistry: Ceramics and Metals, 17th R isø InternationalSymposium und Material Sience, Roskilde, Denmark, September 1996. Thisprocess is also suitable for complex surface geometries, in particularfor the curved surfaces which are present in a cylindrical or tubularfuel cell. However, the EVD process is very expensive and complex.

It is known from the article that both the electrolyte layer and theinterconnector and the anode are usually applied by the EVD process. Thearticle deals with the problem of replacing the expensive EVD processwith other coating systems. While new coating processes, for exampleplasma-coating processes, are proposed for the interconnector and theanode, the EVD process continues to be envisaged for the electrolytelayer in order to ensure that a sufficiently high quality is achieved.

One problem with the electrolyte layer is considered to reside inparticular in the gastightness required. The solid electrolyte layercannot generally be formed to be sufficiently gastight with aconventional plasma coating process, which is significantly lessexpensive than the EVD process. A plasma coating process usually appliesa molten or pasty coating material, which then solidifies, to thesubstrate to be coated within a relatively small spray spot with adiameter of up to 4 cm. In the case of large-area components, forexample in the case of the known tubular fuel cells, the small size ofthe spray spot means that the coating has to be applied in a pluralityof tracks.

A plasma coating process of this type throws up in particular twoserious problems for the formation of a solid electrolyte layer:firstly, the molten material shrinks as it solidifies, so that aporosity which corresponds to the degree of shrinkage is formed in thesolid electrolyte layer. Therefore, a gastight layer cannot be achieved.Secondly, a process of this type is scarcely able to achieve homogeneousformation of the solid electrolyte layer with a uniform thickness, sincethe individual tracks overlap at their boundary regions. Thefluctuations in thickness in the region of the solid electrolyte layersignificantly reduce the efficiency of a fuel cell, since the ionconductivity across the solid electrolyte layer is adversely affected.

On account of the significant cost benefits of a plasma coating process,for example compared to the EVD process, more recent developments alsoaim to use plasma coating processes for the solid electrolyte layer.This is shown, for example, by the scientific article by Rudolf Henne“Potential of Vacuum Plasma Spraying for the Production of SOFCComponents” and the article by Heiko R. Gruner et al. “SOFC Elements byVacuum-Plasma-Spraying (VPS)”, both included in “Proceedings of firstEuropean Solid Oxide Fuel Cell Conference”, Lucerne, Switzerland,October 3 to 7, 1994, Volume 2, pages 617 to 627 and pp. 611 to 616,respectively. A common feature of both these articles is that theypropose a vacuum plasma process, with a pressure of approximately 100mbar being established in the coating chamber.

FR 2 729 400 A1 and the patent abstract to the Japanese patentapplication 63000450 respectively propose pressures in the range from0.1 to 20 mbar and 10 to 100 torr (13 to 130 mbar).

However, even with these known plasma-coating processes, it isimpossible to achieve a high-quality solid electrolyte layer which hasthe quality of a solid electrolyte layer produced using the EVD process.

SUMMARY OF THE INVENTION

An embodiment of the invention is based on an object of providing aninexpensive and simple process for producing a solid oxide fuel cellwith a high efficiency.

According to an embodiment of the invention, the object may be achievedby a process for producing a solid oxide fuel cell having a solidelectrolyte layer arranged between two porous electrodes, theelectrolyte layer being applied in gastight form to one of theelectrodes by means of a plasma spraying process inside a coatingchamber. A pressure of less than approximately 15 mbar, and inparticular a pressure in the range between 1 and 5 mbar, is establishedin the coating chamber. Individual particles of a coating material areentrained in a plasma jet and are thinly distributed in the plasma jetin such a manner that they are applied to the substrate (electrode) insubstantially isolated form.

A plasma-coating process of this type forms a high-quality solidelectrolyte layer, which is quite comparable to a solid electrolytelayer produced by means of an EVD process, and in particular is veryhighly gastight. In this context, it is essential that the individualparticles be extremely thinly distributed in the plasma jet and beapplied in isolated form. The layer is therefore produced by“punctiform” application of individual particles. The individualparticles are distributed finely and in dispersed form in the plasmajet.

In this context, the term “punctiform” is understood as meaning thelocally limited application of coating material, specifically delimitingthe process with respect to the application of a dense coating materialin conventional processes. The punctiform application is essential forthe formation of a layer with a very low porosity, since during coolingeach individual particle solidifies in a substantially isolated way andtherefore also shrinks in an isolated way. The free space (porosity)which results from the shrinkage of the individual particle is closed upby particles which subsequently impinge on the surface and in turnsolidify in a substantially isolated way. The isolated solidificationtherefore leads to the porosity which inevitably occurs during theprocess being filled up by subsequent coating particles, so that a denselayer is formed.

The isolated application is assisted by the low pressure in the coatingchamber, which allows a relatively wide plasma jet. The widening of theplasma jet results in a very low material surface density for the sameproportion of coating material (particle concentration) in the plasmajet per unit area of the substrate to be coated and makes it possible toachieve a very large coating area. The coating material is thereforeapplied very thinly, with the result that a low porosity is achieved. Atthe same time, a very large area of the surface to be coated is covered,so that single-track application without overlap areas or applicationwith only a few overlap areas becomes possible.

If overlap areas are required, these are not critical at least onaccount of the thin application in terms of their overall thickness.Furthermore, the low pressure in the coating chamber has theadvantageous effect that interaction between the individual particles ofthe plasma jet and the surroundings is substantially prevented, andtherefore a good ion conductivity can be achieved. Moreover, the lowpressure indicates that there are few instances of the individualparticles colliding with one another.

A coating process of this type can be carried out, for example, on thebasis of the plasma spraying process described in U.S. Pat. No.5,853,815. In that document, the process disclosed is referred to as anLPPS (low pressure plasma spraying) thin-film process. In this case, avery low pressure, which is lower than the pressure range ofapproximately 30 mbar which is usually employed in a conventional LPPSprocess, is established in the coating chamber. In this process, aconsiderable temperature difference is established between the interiorof a plasma gun which is used and the outside space. The high pressuredifference helps the jet to widen.

To produce a continuous, i.e. closed layer on the electrode, it ispreferable for the latter to be covered a number of times by the plasmajet. Therefore, a plurality of passes are carried out by the plasma jet,and the “punctiform” coating which takes place in the process closes upthe shrinkage porosity between the individual coating spots in aparticularly advantageous way. The speed at which the plasma jet ispassed over the electrode is therefore sufficiently high to ensure“punctiform” coating.

In order to form a particularly dense layer, the individual coatingspots formed during one pass are so thinly distributed that the layeradvantageously only becomes continuous after the plasma jet has passedover the electrode 20 to 60 times. The then continuous layer preferablyhas a layer thickness of 5 μm to 10 μm.

In an advantageous configuration, the overall layer thickness of thesolid electrolyte layer is approximately 30 μm. After approximately20–60 passes of the plasma jet, the “punctiform” coating produces acontinuous layer with the abovementioned layer thickness of from 5 to 10μm. Thus, between 60 and 400 passes are carried out to form theelectrolyte layer.

If the coating is effected by application in tracks, the overlap regionsof the individual layers are advantageously offset with respect to oneanother, so that a homogeneous overall thickness of the solidelectrolyte layer is produced.

On the electrode, the plasma jet preferably has a jet diameter ofbetween 30 and 50 cm, in particular a jet diameter of 40 cm. Compared toa conventional process with a plasma jet diameter of typically 4 cm.Thus, the plasma jet area is increased by a factor of 100.

To form a particularly dense layer, a pulverulent coating material witha mean particle diameter of less than 20 μm is introduced into theplasma jet. It is in this case preferable for the mean particle diameterto be less than 10 μm. The preferred particle diameter is well below 10μm. In the present context, the term mean particle diameter isunderstood as meaning what is known as the D₅₀ value, which denotes that50% of the particles have a diameter of less than, for example, 10 μm(for a mean particle diameter of 10 μm). The small particle diametersallow considerable melting of the individual particles in the plasmajet.

The plasma-coating process preferably produces an electrolyte layerwhich has a leak rate of less than approximately 10*10⁻⁴ mbar l/sec/m².In particular, the leak rate is below 2.3*10⁻⁴ mbar l/sec/m². A leakrate of this type is therefore comparable to that which can be achievedby use of an EVD process. Therefore, a solid electrolyte layer of thesame good quality can be formed using the plasma coating process, whichis highly expedient compared to an EVD coating.

According to a particularly preferred embodiment, the density of thecoating, and therefore its porosity, is set by varying the processparameters. This makes it possible to provide layers of differentdensity using the same plasma-coating process. In particular, they mayalso be layers of different chemical composition. In this case, thepulverulent coating material which is introduced into the plasma jet isvaried.

It is preferable for the density to be set by selecting the meanparticle size. The other process parameters, in particular the lowpressure in the coating chamber, therefore remain substantiallyidentical. To produce a porous layer, by way of example particles with amean particle diameter of >10 μm and in particular with particlediameters in the region of 45 μm are used. Such large particles scarcelymelt or melt only slightly. Overall, it is possible to establish aporosity of between approximately 0 and 10%, based on the volume of thelayer, by selecting the particle size. The process therefore enables aplurality of layers of the solid oxide fuel cell to be produced in asimple way.

It is preferable for an interconnector to be applied as a further layerby way of the plasma-spraying process. An interconnector of this type isgenerally provided in tubular fuel cells and is used for connection ofthe cathode, which is cylindrically surrounded by the anode. Theinterconnector is conductively connected to the cathode.

It is also preferable for at least one of the porous, solid oxideelectrodes to be produced by use of the plasma spraying process. Forthis purpose, in particular the particle diameters mentioned above, inthe region of 45 μm and above, are used. The advantage of theconsiderable jet widening in the plasma coating process and the build-upfrom layers produced in “punctiform” fashion continues to apply withregard to the formation of the porous electrode. The size of theindividual particles, which are scarcely melted, indicates that cavitieswhich form the porosity remain between the individual particles.

According to a particularly expedient configuration, the processparameters are varied during the coating. As a result, it is possible toapply different layers in a quasi-continuous coating operation. Inparticular, it is possible to apply layers which have a gradient interms of the chemical composition and/or the porosity. Therefore, theprocess makes it possible to use a continuous coating process to producealmost the entire fuel cell, comprising two electrodes, the solidelectrolyte layer and the interconnector, if the process is carried outin a suitable way.

The process is suitable in particular for a fuel cell which is designedas a tubular hollow body.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention is explained in more detailbelow with reference to the drawings, in which, in each case indiagrammatic form:

FIG. 1 shows a cross-sectional view through a tubular fuel cell,

FIG. 2 shows a structure for carrying out the plasma coating process,and

FIG. 3 shows a structure for determining the leak rate of the coatingapplied.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with FIG. 1, a solid oxide, tubular fuel cell 2 includes afirst electrode, which is referred to as cathode 4, and a secondelectrode, which is referred to as anode 6. The cathode 4 and the anode6 are designed as cylinders arranged concentrically with respect to oneanother and consist of a porous ceramic material. A fuel cell 2 of thistype with solid oxide electrodes is also known as an “SOFC fuel cell”(solid oxide fuel cell). Air or oxygen is passed through the innertubular cathode 4, while a fuel, for example hydrogen, is guided pastthe anode 6 surrounding the cathode 4. The cathode 4 is also known asthe air electrode and the anode 6 as the fuel electrode. A solidelectrolyte layer 8 is arranged between the cathode 4 and the anode 6.

On the one hand, the solid electrolyte layer 8 has to be gastight, inorder to prevent contact between the air and the fuel via the porouselectrodes 4, 6. At the same time, the solid electrolyte layer 8 has tohave a good ion conductivity, so that when the fuel cell is operatingions can migrate between the cathode 4 and the anode 6 in order to buildup a voltage. To achieve a good ion conductivity between the twoelectrodes 4, 6, it is preferable for a special interlayer to be placedbetween the solid electrolyte 8 and the anode 6.

Neither the solid electrolyte layer 8 nor the anode 6 form a completering. Rather, they have an opening in the ring, in which what is knownas an interconnector 9 is arranged and is directly connected in anelectrically conductive manner to the cathode 4. The cathode 4 on theinner side can be electrically connected via the interconnector 9.

To achieve the highest possible efficiency of the fuel cell 2, highdemands are imposed on the solid electrolyte layer 8 in terms of itsgastightness and its ion conductivity. Therefore, only processes whichare able to produce both a dense solid electrolyte layer 8 and a veryhomogenous solid electrolyte layer 8 with a constant layer thickness aresuitable for application of the solid electrolyte layer 8. To satisfythese demands, the solid electrolyte layer 8 has usually been applied bymeans of the expensive EVD coating process (EVD: electron vapordeposition).

The basic structure for carrying out a special plasma spraying process,known as the LPPS thin-film process, is illustrated in FIG. 2. Amultiplicity of fuel cells 2 which are to be coated are arranged in acoating chamber 10. These fuel cells 2 are introduced into anddischarged from the coating chamber 10 via locks 12. Each individualfuel cell 2 can be rotated about an axis of rotation 14, which isindicated by dashed lines, in order to enable them to be coateduniformly and on all sides. What is known as a plasma gun 16 is arrangedin the coating chamber 10 in order to carry out the plasma coatingprocess. A plasma is usually produced in this plasma gun.

To control and supply the plasma gun 16, there is a control and supplyunit 18 which is connected to the plasma gun 16. A powder as coatingmaterial is preferably introduced into the plasma jet 20 produced in theplasma gun 16 from a powder source 22, preferably directly at the outletof the plasma gun 16. The control and supply unit 18 is connected to thepowder source 22 and controls the rate at which the powder is fed intothe plasma jet 20.

Conventional plasma coating processes are unsuitable for applying thesolid electrolyte layer 8, since they do not allow either a sufficientdensity or a constant layer thickness of a large-area coating to beachieved. To form a high-quality solid electrolyte layer 8, a pressureof less than 15 mbar, and in particular less than 5 mbar, is establishedin the coating chamber 10. At the same time, the powder feed rate isselected in such a manner that the individual powder particles strikethe surface to be coated in isolated form.

On account of the low pressure in the coating chamber, the plasma jet 20widens out very considerably after it leaves the plasma gun 16. Thisassists the very fine, dispersed distribution of the powder over thecoating surface 24. In the case of the thin-film process, the coatingsurface 24 typically has a jet diameter D of approximately 40 cm. As aresult, large areas can be homogeneously coated with very thin layers.

Solid electrolyte material is used as powder from the powder source 22for the application of a solid electrolyte layer 8. To produce asufficiently dense layer, a powder with a mean particle diameter of lessthan 20 μm is used. In particular, particle diameters of less than 10 μmare used.

The considerable beam widening, caused by the low pressure in thecoating chamber, the appropriate powder feed rate and the fineparticles, advantageously lead to an extremely dense solid electrolytelayer being formed. Its leak rate is typically less than 2.3*10⁻⁴ mbarl/sec/cm² and is therefore comparable to the leak rate which can beachieved by use of the EVD process.

The dispersed distribution of the fine particles over the coatingsurface 24 is crucial to achieving such a low leak rate. The individualparticles strike the surface which is to be coated in, as it were,isolated form, without forming lumps (clusters). On account of theirsmall diameter, the particles have been partially melted in the plasmajet and are in predominantly molten form. After they have struck thesurface, they solidify, with their volume being reduced in the process.In conventional processes, in which the particles are not applied to thesurface to be coated in dispersed form, but rather in large numberssimultaneously as a pasty material, this shrinkage on solidificationleads to a porosity being formed and prevents a dense layer from beingformed. Although the individual particles also shrink in the plasmaspraying process according to the invention, the open volumes producedby the shrinkage are closed up tightly by particles which subsequentlystrike the surface.

To achieve uniform coating of the entire fuel cell 2, the plasma gun 16is arranged in such a manner that it can move in the directionsindicated by the arrows 26 in the coating chamber 10. At the same time,the fuel cells 2 are rotated, so that homogenous and uniform applicationis achieved.

It has been found that particularly good results can be achieved with apressure of 1.5 mbar in the coating chamber 10, a plasma gas flow rateof 120 SLPM (standard liters per minute) and a powder delivery rate of80 g/min. The plasma gas used is in particular argon.

To form the solid electrolyte layer 8, a plurality of individual layersare successively formed by “punctiform” coating (each particle strikesthe surface to be coated separately in a pasty state, shrinks separatelybefore being struck by another particle). This is done until the desiredtotal layer thickness of the solid electrolyte layer of approximately 30μm is reached.

A critical advantage of the process resides in the fact that theproperties of the coating applied can be varied within a wide range byvarying the process parameters. This process is therefore suitable notonly for the application of the solid electrolyte layer 8 but also, inparticular, for the application of the interconnector 9, which inparticular is also formed to be gastight. For application of theinterconnector 9, a suitable coating material for the interconnector 9is introduced into the powder source 22. It is advantageous for aplurality of powder sources 22—not shown in FIG. 2—holding differentcoating materials to be provided in the coating chamber 10.

However, in addition to the interconnector 9, it is also possible forthe electrodes, in particular the anode 6, to be applied to the solidelectrolyte layer 8 by means of the LPPS thin-film process. For thispurpose, suitable ceramic powders are used as coating material. Sincethe anode 6 has to be porous, suitable ceramic powders with particlediameters of in particular greater than 45 μm are used. In general, aporosity of up to 10%, based on the volume of the coating, can beachieved in the layer forming by suitable selection of the particlesize. This is attributable to the fact that large particles do not meltor only melt incompletely in the plasma jet 20 and are therefore appliednext to one another as small balls, as it were, with cavities includedbetween them. The process described is therefore particularlyadvantageously suitable for use in the field of fuel cells 2, since thedifferent coatings for a solid oxide fuel cell 2, in particular atubular solid oxide fuel cell 2, can be carried out quickly andinexpensively using a single process.

FIG. 3 shows the structure used to determine the leak rate of thecoating applied. In accordance with DIN 28402, the pV flow of a gasthrough the coating is determined in order to establish the leak rateq_(L). A specific differential pressure Δ is applied between the outerside and the inner side of the coating. The leak rate q_(L) isdetermined according to the following formula:q _(L) =dp/dt*V,where dp is the pressure rise or pressure drop in the measurement timedt in the volume V.

To determine the leak rate q_(L), the fuel cell 2 to be tested isintroduced into a special adapter 28. The adapter 28 has a window 30with a defined area and completely seals off the fuel cell 2 withrespect to the environment, apart from the window 30. Therefore, theouter layer of the fuel cell 2 is in contact with the ambient pressurevia the window 30. Air is pumped out of the interior of the fuel cell 2via a first valve 34 a and a second valve 34 b via a pump line 32, and asubatmospheric pressure is generated. For this purpose, a pump 36 isprovided. When a set subatmospheric pressure is reached, the valves 34a, 34 b are closed. Then, the temporal curve of the pressure rise on thebasis of the leak area defined by the window 30 is recorded by means ofa pressure gauge 38. This pressure rise is transmitted from the pressuregauge 38 to an evaluation unit 40, where it is evaluated. A furthervalve 34 is provided, in order to vent the individual lines after themeasurement cycle has ended.

To determine the leak rate q_(L), the procedure is as follows: thepressure rise is measured over a measurement time dt of 30–600 sec. Thedifferential pressure Δ is 1 bar, the measurement takes place at roomtemperature and the test gas is air. With a measurement structure ofthis type, a leak rate of less than 2.3*10⁻⁴ mbar l/sec/cm² is achievedfor a solid electrolyte layer which has been applied using the processdescribed above.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. A process for producing a solid oxide fuel cell, including a solidelectrolyte layer arranged between two porous electrodes, comprising:applying the solid electrolyte layer in gastight form to one of theelectrodes via a plasma-spraying process in a coating chamber in whichthe pressure is less than 15 mbar, wherein individual particles of acoating material are entrained in a plasma jet, and wherein theparticles are thinly distributed in the plasma jet in such a manner thatthey are applied to the one of the electrodes in substantially isolatedform.
 2. The process as claimed in claim 1, further comprising:producing a continuous layer after the plasma jet has passed over theelectrode a plurality of times.
 3. The process as claimed in claim 2,wherein the layer is continuous after the plasma jet has passed over theelectrode about 20 to 60 times.
 4. The process as claimed in claim 1,wherein the total layer thickness of the solid electrolyte layer isbetween 5 μm and 30 μm.
 5. The process as claimed in claim 1, whereinthe plasma jet on the electrode includes a jet diameter of between 30and 50 cm.
 6. The process as claimed in claim 1, wherein a pulverulentcoating material with a mean particle diameter of less than 20 μm isintroduced into the plasma jet.
 7. The process as claimed in claim 1,wherein the leak rate of the solid electrolyte layer is less than10*10⁻⁴ mbar l/sec/cm².
 8. The process as claimed in claim 1, whereinthe density of the coating is set by varying the process parameters. 9.The process as claimed in claim 8, wherein the density is set byselecting the mean particle size.
 10. The process as claimed in claim 1,further comprising: applying an interconnector as a further layer viathe plasma-spraying process.
 11. The process as claimed in claim 1,wherein at least one of the electrodes is produced via theplasma-spraying process.
 12. The process as claimed in claim 1, whereinthe process parameters are varied during the coating.
 13. The process asclaimed in claim 1, wherein the fuel cell is designed as a tubularhollow body.
 14. The process as claimed in claim 3, wherein the layerhas a layer thickness of 5 μm to 10 μm.
 15. The process as claimed inclaim 1, wherein the total layer thickness of the solid electrolytelayer is between 5 μm and 30 μm.
 16. The process as claimed in claim 1,wherein the total layer thickness of the solid electrolyte layer isbetween 5 μm and 30 μm.
 17. The process as claimed in claim 4, whereinthe plasma jet on the electrode includes a jet diameter of 40 cm. 18.The process as claimed in claim 2, wherein the plasma jet on theelectrode includes a jet diameter of between 30 and 50 cm.
 19. Theprocess as claimed in claim 3, wherein the plasma jet on the electrodeincludes a jet diameter of between 30 and 50 cm.
 20. The process asclaimed in claim 1, wherein a pulverulent coating material with a meanparticle diameter of less than 10 μm is introduced into the plasma jet.21. The process as claimed in claim 1, wherein the leak rate of thesolid electrolyte layer is less than 2.3*10⁻⁴ mbar l/sec/cm².
 22. Amethod for producing a solid oxide fuel cell, including a solidelectrolyte layer arranged between two porous electrodes, comprising:applying the solid electrolyte layer in gastight form to one of theelectrodes via a plasma-spraying process in a coating chamber in whichthe pressure is less than 15 mbar, wherein particles of a coatingmaterial are dispersed in the plasma jet in such a manner that they areapplied to the one of the electrodes in substantially isolated form. 23.The process as claimed in claim 22, further comprising: producing acontinuous layer by passing the plasma jet over the at least oneelectrode a plurality of times.
 24. The process as claimed in claim 23,wherein the layer is continuous after the plasma jet has passed over theelectrode about 20 to 60 times.